The present invention relates to a hydraulic vehicle servo brake with a master cylinder comprising at least one stepped main-piston assembly, with an annular chamber provided at the annular step of the main piston, with a booster piston positively engaged with the main piston and being adapted to be acted upon by the brake pedal and by the controlled pressure that is supplied by a brake valve governed by the brake pedal. A first annular seal seals the booster piston in relation to the control pressure chamber, with a second annular seal sealing the main piston in relation to the annular chamber, as well as with a valve disposed in the annular chamber. When the brake pedal is not depressed, the valve disposed in the annular chamber is opened due to the main piston being in its inactive position and connects the annular chamber with the supply reservoir and which will close already in the event of a slight advance movement of the main piston. After closing of the valve, the annular chamber will be connected to the pressure chamber of the main piston and a wheel slip brake control unit will shut off the annular chamber as soon as wheel slip occurs. An annular gap closed by two annular seals and connected to the supply reservoir is arranged at the enlarged main piston part so that the annular chamber is connected to the supply reservoir port in the presence of a pedal travel which is still sufficient for performing a panic stop that is not boosted by auxiliary force. In this arrangement, the two annular seals are secured to the master cylinder.
In such prior art vehicle servo brakes (German printed and published patent application 33 38 249), the main piston and the booster piston are two different components, their facing end surfaces including coaxial recesses in which a tappet is arranged providing positive engagement between the two pistons.
The diameter of the booster piston is larger than the diameter of the tapered main piston part, to the end that, in consequence of a predefined controlled pressure in the control pressure chamber, approximately the same amount of pressure will be attained in the main piston pressure chamber. However, the enlarged main piston part is of somewhat larger diameter than the booster piston so that the pressure in the main piston pressure chamber is correspondingly somewhat less than the controlled pressure. This slight difference of surfaces between the enlarged main piston part and the booster piston is important for the reason that, when the controlled pressure is applied on the main-piston pressure chamber, a resultant resetting force is caused to act on the main-piston/booster-piston arrangement in the event of wheel slip, when the annular chamber is connected to the supply reservoir upon a too great advance movement of the main piston. This way, a reliable resetting of the main piston is accomplished until a position is reached at about 40% of the total pedal travel. Accordingly, consumption of pressure fluid which is considerable under certain circumstances upon the occurrence of wheel slip will not result in the main piston being advanced beyond the 40% pedal position. This is important in order to have available at any time sufficient pedal travel for a panic stop that may become necessary, for instance upon failure of the wheel slip brake control unit.
The annular chamber which encompasses the booster piston and the tapered main piston part in the area of their opposing end surfaces communicates with the supply reservoir by way of a supply-reservoir connecting bore, so that this chamber is always unpressurized. This is significant because, in the event of the first or the second annular seal becoming leaky, pressure fluid can discharge into the supply reservoir more or less unhindered, in consequence whereof the brake pedal will be depressed through its full travel, and malfunction of the brake will occur.
It is an object of the present invention to create a hydraulic vehicle servo brake of the type described which is of more compact design and which will function with a reduced volume consumption of the brake booster in order to reset the main piston in the event of beginning wheel slip.